First, PCI and PCI Express are two different things. PCI video cards are an older standard and generally, aren't found anymore. AGP is still widely used, but PCI Express is the "new hotness" for video. Depending on how old your motherboard is, it may only have an AGP slot, and not PCI Express. They are different in size, so make sure you know which slot your motherboard has first before you buy; otherwise you may have a card that you cannot use.
AGP's bandwidth on the system bus is between 256 MB and 2 GB, depending on if you use AGP 1X, 2X, 4X, or 8X. Again, this is dependent on what your motherboard will support. PCI Express has a higher bus bandwidth that ranges between 512 MB and 8 GB, in duplex mode.
Hope this helps!